This application claims the priority of German Patent Document 100 49 660.1, filed in Germany, Oct. 7, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for the production of locally reinforced sheet-metal mouldings. Preferred embodiments relate to such sheet-metal mouldings as structural parts of motor vehicle bodies. German Patent Document DE 43 07 563 A1 also relates to such structural parts.
With a view to saving weight on motor vehicle bodies, it is known to provide locally restricted reinforcing sheets at force application points in sheet-metal structural parts of bodies, so that the entire structural part does not have to be produced from a thick sheet. To increase the flexural resistance of hollow members formed from sheet-metal shells, reinforcing sheets can be integrated, on the inside or outside, on at least one of the sheet-metal shells, these reinforcing sheets extending over a relatively long region of the hollow member. For stiffening purposes, stiffening tubes of constant cross section have also already been embedded inside the hollow members, but, although approximately following the profile of the hollow member, do not completely fill the cross section of the latter, instead being connected via web plates or directly to the wall of the hollow member at specific points only. Structures of this type for increasing the rigidity of a hollow member have been provided, for example, on the center pillars of passenger car bodies for a side-on crash situation. It is usually advantageous, again for reasons of weight, at the joints or connection points, that is to say at the intersections of hollow members, to integrate a locally restricted reinforcement into the wall of the transverse hollow member.
Previously, the basic sheet and the reinforcing sheet were in each case deep-drawn separately and these part-workpieces were subsequently welded together to form a complete structural part, thus presupposing two separate drawing and forming operations, two separate sets of tools and a separate operation for the assembly of three-dimensionally shaped sheet-metal parts. All this is time-consuming and costly.
In the method, known from German Patent Document DE 43 07 563 A1, for the production of locally reinforced sheet-metal structural parts for motor vehicle bodies, a sheet billet forming the basic sheet of the structural part is connected locally, that is to say at a predetermined point, in the flat state to a smaller reinforcing sheet of preferably higher strength, and this largely planar composite sheet structure is subsequently jointly deep-drawn in a press. The reinforcing sheet may itself be partially preformed, for example be provided with stiffening beads. The reinforcing sheet is so designed in its contour profile and is so positioned on the basic-sheet billet that, after deep-drawing, the reinforcing sheet optimally fills the force application point. Depending on the application, the reinforcing sheet may have a higher strength than the basic sheet, for which purpose a greater wall thickness of the reinforcing sheet and/or a better material quality may be provided. Under some circumstances, a multi-layer design of the reinforcing sheet is also recommended, the plurality of layers preferably having an extent of differing size.
As techniques for the connection of the basic-sheet billet and the reinforcing sheet, German Patent Document DE 43 07 563 A1 refers, inter alia, to setting or pressing assembly (so-called clinching), this being considered particularly advantageous because this assembly technique can intrinsically be integrated into the forming operation or into the affixation of any fastening elements—punch-in nuts, punch-in bolts. In this case, under some circumstances, the reinforcing sheet merely needs to be placed in a defined position after the basic sheet in the forming press. In addition, welding connection techniques and adhesive bonding techniques are also mentioned, and, with regard to these connection techniques, it is stressed, inter alia, that they leave the surface of the structural part unaffected. It is plainly recognized in the publication mentioned that the mutual contact surfaces of the basic sheet and reinforcing sheet must be protected against corrosion, because subsequent vehicle painting or wax preservation of these surfaces cannot permanently protect against the ingress of moisture. It is therefore recommended to use sheets galvanized at least on one side for the basic sheet and the reinforcing sheet or, instead, to insert a protective film made of plastic or metal at the contact point. The corrosion-preventing action of any adhesive layer is also emphasized.
Admittedly, as compared with the procedure outlined above, with a separate forming of the basic part and the reinforcing part, the method known from German Patent Document DE 43 07 563 A1 is substantially more cost-effective because the deep-drawing operation is combined in a single uniform work cycle. However, disadvantages are, on the one hand, that very high forming forces are necessary for this purpose, particularly when the reinforcing sheet has a higher strength or even itself has a multi-layer design. This is manifested in correspondingly higher stress and therefore in a higher elastic deformation of the individual parts of the forming press, thus, in turn, leading to lower working accuracy and a shorter useful life of the forming tools and the press. It is true that the higher stress or the higher elastic deformation of the press parts can be compensated by a corresponding dimensioning of the latter, although this increases the costs of the press. However, the increased tool wear cannot be compensated and therefore likewise adds to the piece costs of the structural parts produced.
In the already known technique, assessed here, of the common forming of patched composite sheets, it must be remembered that the degrees of reinforcement capable of being achieved thereby are only limited, since the forming and drawing forces of the reinforcing sheet can be transmitted into the latter to a substantial extent only indirectly via the basic sheet and via the connection points between the basic sheet and the reinforcing sheet which are affixed peripherally to the reinforcing sheet. If, whether because of the higher material strength or because of the greater wall thickness or on account of both measures, the reinforcing sheet is substantially more resistant than the basic sheet, it cannot be formed in any desired way jointly with the basic sheet. If particularly high degrees of reinforcement are to be implemented locally, the basic sheet and the reinforcing sheet must in each case be formed separately and the two parts subsequently welded to one another to form a composite part.
Regardless of whether the reinforcing sheet is formed jointly with the basic sheet to form a structural part or whether, as mentioned in the introduction, the basic sheet and the reinforcing sheet are in each case formed separately and two mouldings are only subsequently assembled to form the structural part, it is particularly important, in view of the intended reduction in the wall thickness of the basic sheet for reasons of weight and the consequently likewise reduced corrosion reserve, to have effective corrosion protection at the contact point of the basic sheet and reinforcing sheet, to avoid the risk of a reduced useful life of the structural part. Effective corrosion protection at the contact point of the sheets by means of the method known from German Patent Document DE 43 07 563 A1 cannot be reliably ensured even by corrosion protection films being interposed. To be precise, an interposed corrosion protection film is destroyed locally by the punctiform connection points (clinches or spot welds) between the basic sheet and reinforcing sheet, so that moisture creeping in by capillary action may lead to corrosion precisely at the connection points and in time may pit the relatively thin basic sheet there. In spite of painting and wax preservation, the ingress of moisture to the contact surface between the basic sheet and the reinforcing sheet must be expected, even in the protected inner region of bodies, because of the formation of perspiration water and on account of capillary action.
On the basis of the above assessment of the known method according to German Patent Document DE 43 07 653 A1, the weak points of this method which are inherent in the system may be summarized as follows:    limited degrees of forming of the sheets jointly to be formed;    a high recovery behavior of the jointly formed sheets;    because of this recovery behavior, comparatively low dimensional accuracy of the structural parts;    due to the indirect transmission of forming forces into the reinforcing sheet, only limited degrees of reinforcement can be implemented at the reinforcing points;    high pressing forces during joint forming and therefore high investment costs;    high tool wear and therefore an increase in piece costs;    limited corrosion protection on the assembly surface.
DE 195 29 881 C1 discloses the production of a deep-drawn part from a sheet of hardenable spring steel, the spring steel being deep-drawn in the warm state of the sheet and, in the ready-formed state, being brought to spring steel hardness by subsequent heat treatment, although there is no mention of where and how this takes place. Heat treatment subsequent to deep-drawing entails the risk that the parts will warp and therefore be subject to a pronounced dimensional and/or shape-related spread. Apart from this, in the case of locally sharply varying stress on the components, the wall thickness must be designed for the point where stress is greatest, thus leading to an overdimensioning of the wall thickness at points where stress is relatively low. It would be conceivable, in theory, subsequently to reinforce this moulding locally by the welding-in of a reinforcing sheet. However, this would not be expedient since the hardness of the basic sheet and/or that of the reinforcing sheet would be lost locally at the welds. It would be appropriate, if the sheets involved had high strength, to rule out other connection techniques, such as soldering, adhesive bonding or clinching, because these connection techniques would not be feasible without the basic sheet and/or the hardened material structure being impaired (soldering, clinching) or because they would not afford the necessary strength (adhesive bonding). The connection points would have to have at least approximately the same strength as that of the hardened basic sheet, in order to achieve the desired reinforcing effect of the embedded sheet. The known method according to German Patent Document DE 195 29 881 C1 therefore has the following weak points, and reference may be made partially to the statements regarding the literature reference mentioned in the introduction:    only light-weight construction possible because of a uniform wall thickness;    no local reinforcements possible;    comparatively low dimensional accuracy of the structural parts because of the heat treatment.
An object of the invention is to improve the generic production method in terms of one or more of the following criteria:                With regard to the product of the method, to the effect that higher local degrees of reinforcement can be achieved in a unitary manufacturing step, in comparison with the basic sheet or with the unreinforced component.        As regards the result of the method, to the effect that a higher working accuracy of the mouldings than hitherto can be achieved.        As regards the means for carrying out the method, to the effect that the forming tools and the forming press are subject to lower stress than expected and, to that extent, a justifiable useful life can be expected.        
This object is achieved according to certain preferred embodiments of the invention, by providing a method for the production of a three-dimensionally shaped structural part comprising a basic sheet and at least one smaller locally arranged reinforcing sheet, in which the basic sheet is connected in a flat state or in an incompletely formed preforming state, to a reinforcing sheet at the point predetermined for a subsequent reinforcing point, and the parts of the patched composite sheet structure are subsequently jointly formed by an openable and closeable forming tool in a forming press, wherein the patched composite sheet structure is heated before joint forming to a temperature which is above a forming temperature of the material, is formed in a hot state into a desired shape and is subsequently cooled in the forming tool, which is kept closed, or in a following fixing tool, with the desired forming state being fixed mechanically.
In accordance with preferred embodiments of the invention, the patched composite sheet is formed in the warm state into the desired shape and is cooled in a defined manner on the forming tool, with the forming state being maintained mechanically.
The invention affords a cost-effective way of obtaining a novel light-weight construction concept. To explain this, it may first be recalled that a low weight of a body co-determines essentially the performance (driving enjoyment) and fuel consumption (cost level) of the vehicle and is therefore directly to the customer's benefit. On the other hand, for reasons of high occupant safety, a vehicle body should be as rigid as possible in the region of the passenger cell, the effect of this tending towards a heavier body. These two contradictory requirements as regards weight are, moreover, in direct relation to the costs of the body. The costs, on the one hand, and the weight, on the other hand, tend to develop in opposition to one another; the lighter the design of the body is, the more costly it is, as a rule, to produce, whereas a body construction which exhausts the range of available light-weight construction measures to a lesser extent can be produced more cost-effectively. However, within this network of relations between the function, the costs and the weight of a body, the hot forming according to the invention of structural parts made of steel sets new boundary conditions which make light-weight construction possible at moderate cost, whilst at the same time increasing its functioning. The potentials and advantages of the production method according to the invention are summarized as follows.
A cost-effective light-weight construction becomes possible.
Materials with high and the highest possible strength can be used. The material strength capable of being achieved can be increased about three times, as compared with conventional maximum values.
Despite the use of high-strength materials, high degrees of forming can readily be implemented.
Any desired degrees of reinforcement of the basic sheet can be implemented within wide limits.
The conventional sheet-metal shell construction and the design and repair know-how connected with this can be maintained. The latter assumes great infrastructural importance.
In so far as reinforcements still become necessary, these can be integrated into one shell and into the production of the latter.
Due to the hot forming, the recovery behavior of the sheet-metal parts after the forming process is negligible, thus increasing the manufacturing accuracy. Low manufacturing tolerances can readily be implemented.
The outlay in terms of co-ordination between the forming tool and the structural parts is no higher, but instead lower than in the forming of simple sheet-metal parts.
The costs of plant and tool investments, of the machine hours necessary and of logistics and stockkeeping are reduced.
This list of advantages according to the invention contains some which constitute a synergetic excess with respect to a sum of advantages taken together from different sources of the prior art, specifically the following are to be particularly emphasized as synergetic advantages:                Despite the use of high-strength materials, high degrees of forming can readily be implemented.        Any desired degrees of reinforcement of the basic sheet can be implemented within wide limits.        Due to the hot forming, the recovery behavior of the sheet-metal parts after the forming process is negligible, thus increasing the manufacturing accuracy. Low manufacturing tolerances can readily be implemented.        
So that the structural parts produced can be protected more effectively against corrosion in the contact region between the basic sheet and the reinforcing sheet, in an expedient embodiment of the invention the contact surface of at least one of the sheets is provided in a surface-covering manner with a hard solder before the reinforcing sheet is affixed to the basic sheet, advantageously both contact surfaces previously being cleaned. During the heating of the patched composite sheet structure to forming temperature, this preferably taking place in a protective-gas atmosphere of a furnace, the applied hard solder is melted. During the forming process, an intimate and pore-free soldered joint is produced by the sheet-metal parts being deformed and pressed together. The hard solder, which fills the contact zone in a surface-covering manner, later reliably prevents the ingress of moisture and affords highly effective corrosion protection. The hard solder is preferably applied with an excess which is expressed at the edge of the reinforcing sheet during forming. As a result of the soldering, the step of connecting the reinforcing sheet and the basic sheet is readily integrated into the forming process. It is merely necessary, for example by means of a single connection point placed after the application of the solder, but before heating, between the reinforcing sheet and the basic sheet, to ensure that, during handling, the reinforcing sheet and the basic sheet unequivocally maintain their mutual desired position and, for example, cannot slip out of place in relation to one another.
Further expedient embodiments of the invention may be gathered from the following description and the claims.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.